The invention relates to a sensor head for film measurements, comprising a guide surface for the film, at least one sensor element integrated in the guide surface for measuring a property of the film, and a generator for generating an air cushion between the guide surface and the film, said generator comprising a system for supplying a pressure medium, and a guide member that is made of a porous material and forms at least a part of the guide surface and is adapted to distributedly supply the pressure medium into the air cushion.
A sensor head of this type is used for example in the process of manufacturing blown films, where it is desired to continuously measure the thickness of the film that has been extruded and inflated to a bubble with internal air during the process, so that the conditions of extrusion and cooling may be feedback-controlled such that the film assumes a constant thickness that is uniform throughout the periphery of the film bubble. To that end, the sensor head may comprise one or more optical, radiometric, inductive or capacitive sensor elements with which it is possible to measure the film thickness from one side of the film. An example of a capacitive sensor head has been described in WO 2009/027037 A1.
Preferably, the sensor head revolves on a trajectory around the film bubble so that the film thickness on the entire periphery of the bubble can be measured during one revolution. Moreover, the sensor head should be arranged such that it slightly depresses the film inwardly, so that the film is pressed against the guide surface by the internal pressure of the film bubble. On the other hand, a direct contact between the film and the guide surface should be avoided, especially when the film has not yet been cooled down completely and is somewhat sticky, in order to prevent the film and/or the sensor head from being damaged. For this reason, a pressure medium, e.g. compressed air, is used for generating an air cushion, so that the film may slide over the guide surface without making contact therewith but with a well-defined spacing.
DE 10 2007 034 415 described a sensor head of the type indicated above, wherein the guide surface or at least a part of this guide surface that surrounds the sensor element is formed by a guide member made of a porous material. The compressed air is pressed through the porous material, so that the supply of compressed air is uniformly distributed over the guide surface and, consequently, local distortions of the film are avoided. Otherwise, in some of the known measurement processes, e.g. in case of capacitive measurement with the sensor head described in WO 2009/027037, the measurement result could be corrupted by local fluctuations of the spacing between the film and the guide surface.